1. Field of the Invention
The present invention relates to a color image reading apparatus for reading a color image by using an image sensor.
2. Related Background Art
Hitherto, as an image sensor which is used to read out an image at a high speed, there has been known a type in which a stripe-shaped filter is provided for an image sensor of one line and color separation signals are time sharingly and dot sequentially read out.
The spectral sensitivity characteristic in a low wavelength range is worse than that in a high wavelength range due to the spectral sensitivity characteristic of the color filter which is used to obtain the color separation signals and the spectral sensitivity characteristic of the image sensor itself.
Therefore, an output signal of the image sensor when a reference image of a standard white plate or the like is read does not become an analog video signal which is dot sequentially color balanced. Therefore, to assure a sufficient dynamic range (S/N ratio) of the color separation signals, in an analog video signal processing circuit, the levels of the color separation signals need to be matched. For this purpose, hitherto, in consideration of easiness of circuit construction and stabilization of sampling points when executing A/D conversion, the dot sequential color signals are respectively separated by a sample and hold circuit (hereinafter, referred to as an S/H circuit) and transformed to simultaneous color signals and subjected to signal processes such as amplification and the like. The levels of the color separation signals when the standard white plate was read are matched, thereby adjusting the color balance.
For instance, a color image signal obtained by reading a color original is separated into three color signals of R (red), G (green), and B (blue) by an S/H (sample and hold) circuit as shown in FIG. 1. Therefore, in the construction in which a color original is divided into five parts in the main scanning direction and is read out by a line sensor of five chips, the construction after the color image signals were sampled and held becomes fifteen (=3.times.5) analog signal processing systems. FIG. 1 shows a processing block diagram in which an input color image signal of one channel is sampled and held and amplified in consideration of the white level and black level adjustments and, thereafter, it is returned to the color image signal of one system and is then input to an A/D converter, so that multiplexed digital data OLATCH OUT is obtained. FIG. 2 shows a timing chart of the operation of the circuit in FIG. 1.
As shown in the circuit of FIG. 1, each color separating circuit system has a DC level adjusting circuit in addition to the color balance adjusting circuit. The DC level of a black level signal which was read by the image sensor is shifted, thereby matching the levels among the channels of the image sensor constructed by a plurality of chips and coupling the channels.
However, in the construction shown in FIG. 1, since the dot sequential color signal is separated into simultaneous color signals by the S/H circuit, after the S/H circuit, a number of signal processing circuit systems equal to the number of color separation signals are needed. For instance, in the case of reading an original of, for instance, the A4 size whose width in the longitudinal direction is set to 297 mm, an image sensor of the silicon crystalline type is suitable to read at a high speed. However, in the case of the silicon crystalline type, it is difficult to form a long image sensor of such a type in one chip because of a limitation in manufacturing. Therefore, a plurality of silicon crystalline chips are physically properly arranged to thereby form one line sensor. In this case, there is a drawback such that after the S/H circuit, it is necessary to use signal processing circuit systems of the same type and of the number which is equal to a value in which the number of color separation signals is multiplied with the number of chips constructing the image sensor, so that the circuit construction is extremely large.